English

Cardinal inequalities for $S(n)$-spaces

General Topology 2018-11-01 v1

Abstract

Hajnal and Juh\'asz proved that if XX is a T1T_1-space, then X2s(X)ψ(X)|X|\le 2^{s(X)\psi(X)}, and if XX is a Hausdorff space, then X2c(X)χ(X)|X|\le 2^{c(X)\chi(X)} and X22s(X)|X|\le 2^{2^{s(X)}}. Schr\"oder sharpened the first two estimations by showing that if XX is a Hausdorff space, then X2Us(X)ψc(X)|X|\le 2^{Us(X)\psi_c(X)}, and if XX is a Urysohn space, then X2Uc(X)χ(X)|X|\le 2^{Uc(X)\chi(X)}. In this paper, for any positive integer nn and some topological spaces XX, we define the cardinal functions χn(X)\chi_n(X), ψn(X)\psi_n(X), sn(X)s_n(X), and cn(X)c_n(X), called respectively S(n)S(n)-character, S(n)S(n)-pseudocharacter, S(n)S(n)-spread, and S(n)S(n)-cellularity, and using these new cardinal functions we show that the above-mentioned inequalities could be extended to the class of S(n)S(n)-spaces. We recall that the S(1)S(1)-spaces are exactly the Hausdorff spaces and the S(2)S(2)-spaces are exactly the Urysohn spaces.

Cite

@article{arxiv.1810.12998,
  title  = {Cardinal inequalities for $S(n)$-spaces},
  author = {Ivan S. Gotchev},
  journal= {arXiv preprint arXiv:1810.12998},
  year   = {2018}
}

Comments

13 pages

R2 v1 2026-06-23T04:58:22.071Z